1. Field of the Invention
This invention relates to electronic cameras, and more particularly, to an electronic camera with an accurate white color balance which is automatically set in response to different sources of incident light used to illuminate a subject.
2. Description of the Prior Art
In producing a color image of a subject with a video camera or the like, it is important to determine the color of the light incident upon the subject. So-called "white" light can in fact be of varying colors or color temperatures which a human will subjectively identify as white. For example, a room illuminated by incandescent lamps will appear normal to human vision, and yet the color of the light is more red than brilliant sunlight which is also subjectively identified as normal. Prior art television cameras must be adjusted so that the color white is reproduced as "white" on a monitor, in response to incident light of different colors. Such prior television cameras include means by which the white balance can be selectively adjusted to four discrete settings roughly suitable for four sources of light, such as, brilliant sunlight, cloudy daylight, fluorescent lamps, and incandescent lamps, respectively. Thus, the prior art cameras provide only a coarse or approximate white balance adjustment, and a fine adjustment of the white balance of the displayed color image or picture must be performed by means of a white balance adjustment of a monitoring apparatus such as a video tape recorder, a magnetic disk recorder, or the like.
When a television camera is moved to follow a subject, the white balance necessary to reproduce the color white in a subjectively pleasing manner varies in response to such movement of the subject through shadows and bright sunlight. Since the adjustment with a prior art television camera is only a coarse adjustment, an abrupt and unnatural change occurs in the white balance when the four-position adjustment is changed from one setting to another to compensate for changes in the color of the incident light. It is to be appreciated that the range of changes in the white balance can be quite large when a television camera and portable video tape recorder are used together, making adjustment of the white balance important.
According to one prior art television camera, a light diffusing cap is placed over the lens of the camera and the white balance is adjusted accordingly. As can be appreciated, however, such a camera is inconvenient and cumbersome to use since picture taking must be interrupted to set the camera.
Prior art still cameras which produce video signals from optical images do not generally include means for adjusting the white balance. Of course, a four-way adjustment such as that in a television camera could be included in such a still camera so that the white balance adjustment could be made. However, such an adjustment has not heretofore been provided in a still camera because the camera must be suitable for snapshots and hence, highly portable. The white balance is usually preset for such a still camera to provide easy operation, and hence, the white balance is frequently incorrect. The effect of misadjustment in the white balance is, fortunately, not as pronounced for a still camera as for a television camera, since the still camera takes shots which have more latitude than those taken by a television camera. Nevertheless, it is preferable if the white balance of a still camera is accurately set.